1. Field of the Invention
The present invention relates to a controller of an automatic transmission having a lockup clutch whose closing force is controllable and a control method of such an automatic transmission.
2. Description of Related Art
For a conventional automatic transmission provided with a torque converter which connects an output shaft of an engine and an input shaft of a speed change mechanism through fluid, an automatic transmission provided with the torque converter and a lockup clutch in parallel to transmit force in the driving system efficiently has been disclosed. The lockup clutch is a friction clutch capable of transmitting mechanical force between the output and the input shafts.
Recently, by controlling engagement force (closing force) of the lockup clutch, a control of the lockup clutch controlling a transmitting state of the force at the torque converter, that is, a slip control of the lockup clutch (hereinafter referred to slip control) is carried out.
The slip control is carried out for reducing the slip quantity of the torque converter, reducing the transmission loss of the driving force and reducing fuel consumption.
On the other hand, when the slip quantity of the torque converter is too small, torque vibration generated by an engine is transmitted to the output shaft or torque amplification of the torque converter is not operated, so that the driver feels uncomfortable.
Therefore, in the slip control, the slip quantity between the input shaft and the output shaft of the torque converter is controlled by controlling the closing force of the lockup clutch so that the fuel consumption is reduced and deterioration of the driver can be prevented from feeling uncomfortable.
Further, in the slip control, depending on the driving state, there are two different cases where reduction of the fuel consumption takes precedence over the driver's feelings or where the driver's feelings take precedence over the reduction of the fuel consumption. Therefore, the slip quantity of the torque converter needs to be determined properly at each driving state.
For example, the torque amplification operation of the torque converter is not required in a traveling state with a constant throttle opening, so that reduction of the fuel consumption takes precedence over the driver's feelings and a very small slip quantity is appropriate. On the other hand, when the driver depresses the accelerator, a larger slip quantity is preferable to improve the torque amplification operation of the torque converter.
In Japanese Examined Patent Publication No. Sho 63-13060, for example, target slip quantity is determined in a range to be less than the slip quantity when the driving force is transmitted only by the torque converter and to reduce or stop the transmission of torsional vibration. Further, the closing force of the lockup clutch is controlled to make the difference between the real slip quantity and the target slip quantity smaller.
When the real slip quantity comes close to the target slip quantity, only the engine rotational speed changes and turbine rotational speed which is the output shaft rotational speed of the torque converter does not change. This is because when the vehicle is driven by the engine output, that is, when the engine output is connected to the output shaft of the vehicle, inertia of the input side of the torque converter is smaller than inertia of the output side thereof. Therefore, only the rotational speed in the input side which has the smaller inertia is changed.
Therefore, the target slip quantity is determined and the real slip quantity comes close thereto, that is, the engine rotational speed is determined to be larger than the turbine rotational speed by the target slip quantity with the turbine rotational speed as a reference.
When the speed change is in a transitional period, the change in the turbine rotational speed is not always guaranteed to be constant due to the change in engine torque and the state of the speed change clutch. For example, when the speed change is completed much faster than when it was designed and the adjusting control of the target slip quantity is carried out, the sudden change in the engine rotational speed is required, so that the torque shocks are generated.
In a transmission for separately hydraulic-controlling clutches in the closing side and the opening side, when timings of closing and opening of the clutch do not match, the turbine rotational speed is suddenly increased or decreased, and the transmission moves differently compared with its normal movement at the speed change. In this case, when the adjusting control to the target slip quantity is carried out, an abnormal change in the turbine rotational speed superimposes onto the engine rotational speed, which results in changes in engine sound, vibrations in the tachometer, torque shocks and the like. Thus, the driver feels uncomfortable.
Especially, when the abnormal change in the engine rotational speed occurs before a gear ratio change, the tachometer suddenly moves up or down during the constant traveling of the vehicle, so that the driver feels uncomfortable due to abnormal changes of the throttle operation and the tachometer. On the other hand, when the abnormal change in the engine rotational speed occurs while the gear ratios are changing, that is, during an inertia phase, the tachometer goes up and down although the values are usually reduced together with the gear ratio change, so that the driver feels uncomfortable.